Metal reinforcement is typically used in concrete when the concrete is to be used for structural purposes. The reinforcement acts to both hold the concrete together and significantly increase the strength of the resulting concrete structure. When the most common forms of metal reinforcement is rebar. In a typical process of fabricating a reinforced concrete structure, a rebar grid is fabricated at the site where the concrete is to be poured. Next, a containment form is placed around the rebar grid and concrete is poured over the rebar grid. Once the concrete is cured, the form is removed to reveal a reinforced concrete structure.
As can be appreciated, it is important that individual pieces of rebar and/or the rebar grid do not move appreciably as the concrete is poured over it. Accordingly, the various pieces of rebar comprising the grid are secured to each other to stabilize and strengthen the rebar grid so that it does not move appreciably while it is being encapsulated in concrete. Traditionally, the various pieces of rebar are secured to other pieces of rebar at their respective intersections using steel wire that is wrapped around the rebar pieces and twisted to tension it and hold it in place as shown for example in U.S. Pat. No. 4,096,680.
There are, however, several disadvantages to using steel wire. First, the wire is subject to rust especially when utilized in moist environments. This can be problematic in several ways. If there is a significant period of time between the fabrication of the rebar grid and the pouring of the concrete over the grid, the wire may rust an appreciable amount significantly reducing its strength. The weakened wire can break during the pouring of the concrete thereby permitting the rebar to shift position. Further, any rust on the wire prior to pouring and any rust that forms after pouring while in contact with moisture can leach out of the concrete and cause unsightly discoloration and staining of the resulting structure. While coating the steel wire helps mitigate the problem rust, the coatings are often fragile and can flake off during the application of the wire.
Another problem of using steel wire is that the wire can be significantly weakened as it is plastically deformed while being twisted in place. The extent to which a wire has been weakened may not be readily evidence to an installer and accordingly, the weakened wire can break during the pouring of the concrete. Conversely, if an installer fails to tighten the wire sufficiently for fear of weakening the wire, the associated rebar pieces could move unacceptably during the pouring of the concrete.
Fabricating the rebar grid using wire can be extremely labor intensive as each wire at each intersection of the rebar pieces in the grid must be manually wrapped around the intersection and twisted to hold the rebar in place. Often the amount of space between the various pieces of rebar in a grid is not significant making the process more difficult and time-consuming for an installer. The repetitive action of twisting the steel wires can cause or aggravate carpal tunnel syndrome in the installer as well.
Various solutions to the problems resulting from the use of steel wire have been proposed. U.S. Pat. Nos. 5,699,642 and 6,503,434 teach using plastic tie fasteners, which are more commonly known for use in holding cables together, to hold the rebar together. There are several issues related to the use of these plastic ties. For one, the one way clasps on these plastic ties are not very strong and can loosen or even break as the concrete is poured over the grid. The ties themselves can be relatively weak and subject to stretching as the grid structure sways and moves prior to and during concrete pouring. More stout and stronger tie straps with greater cross sectional areas are often not suitable as they are too stiff to be easily wrapped around the various rebar pieces. Further, the stiffer the strap portions of the ties are, the harder it is to adequately tension the strap by pulling it through the one way clasp member. In short, more flexible smaller plastic tie fasteners do not have adequate strength for securing the rebar pieces of a rebar grid and stouter stronger plastic tie fasteners are too stiff and difficult to work with to be routinely used.
U.S. Pat. No. 4,610,122 teaches another alternative to using steel wire to secure rebar pieces in place. The clip-type device disclosed in this patent essentially comprises to resilient open-sided cylindrical sections that are fixedly secured to each other at right angles relative to their respective longitudinal axis. Operationally, two pieces of rebar that form a 90° intersection are snapped into the respective open-sided cylindrical sections. This solution is also less than ideal. First, this device can only be used when the rebar pieces intersect at a 90° angle. Parallel pieces cannot be joined together. Pieces that intersect act acute angles cannot be joined together. Additionally, if the intersection between two pieces varies more than a few degrees off of 90° as may be expected from time to time in real world construction, an installer may not be able to properly secure the rebar pieces to the device. Second, because the cylindrical sections are opened sided, a rebar piece secured in the device could pop out of the device when subject to particular loads, such as those that can result during concrete pouring.
Another problem associated with any of the aforementioned methods of joining pieces of rebar into a grid is that the pieces of rebar are in contact with each other at the intersection. The recesses of the intersections are places where water can collect, thereby increasing the rate of corrosion and electrolysis of the rebar at that particular location. Additionally, the contact between the two pieces provides a path by which moisture can wick from one rebar piece to another even after the rebar grid has been encased in concrete.